Field of Industrial Use
The present invention relates to a semiconductor element having superior photoelectric conductivity produced by a photochemical gas phase vapor deposition method (hereinafter referred to as an photo CVD for short) and a method of forming the same and various kinds of article in which said element is used. (1) a solar cell having superior long wave length-sensitivity obtained by using said element in a carrier-producing layer; (2) an electrophotographic sensitive member containing said element in a carrier-producing layer; and (3) an image sensor having superior long wave length-sensitivity in which said element is used as a carrier-producing layer.
The present invention is below described in detail for every one of the above described articles. (Prior Art)
Electronic devices utilizing the photoelectromotive force effect are represented by a solar cell. The solar cell, which converts solar energy or other optical energies to electrical energy, is one of the technologies which are being watched with interest as one of countermeasures for energy in the future. The conversion of optical energy to electric energy by a solar cell utilizes the photoelectromotive force effect, which is one of the most fundamental properties of a hetero junction, a pn junction or a pin junction, a Shottkey junction and the like of a semiconductor. The photoelectromotive force effect makes progress in accordance with a mechanism that upon absorption of an incident ray, an electron hole pair is formed and taken out.
However, recently a hydrogenized amorphous silicon a-Si:H is being watched as a material of a thin layer solar cell with interest. This a-Si:H has interesting characteristics in that its absorption coefficient for a light having a wave length near the peak (500 nm) of a solar energy distribution is larger than that of crystalline Si by about one figure; its temperature, at which a thin film is formed, is lower than that of crystalline Si; a film can be directly formed from raw materials by a glow discharge decomposition; also the formation of a junction is easy; and the like.
The most important problem in the design and production of a solar cell is an achievement of a high conversion efficiency. In order to achieve the above described object, a large amount of research and development is being conducted.
Thus, recently an a-SiGe:H solar cell, in which an a-SiGe:H containing Ge added as an additive element is used as an i-layer, has been proposed. As to output characteristics of an a-SiGe:H single-layer solar cell, in which for example an a-SiGe:H film having an inhibit band width E.sub.g .congruent.1.4 eV is used as an i-layer, a short-circuit current J.sub.sc was small to an extent of 9.0 mA/cm.sup.2 and the long wave length-sensitivity was unsatisfactory.
Accordingly, in order to improve the conversion efficiency of an a-Si:H/a-SiGe:H tandem solar cell, it was necessary to improve the characteristics of an a-SiGe:H single-layer solar cell, which was a second layer solar cell, in particular J.sub.sc and the long wave length-sensitivity.
A-SiGe:H film produced by a glow discharge (hereinafter referred to as a GD for short) using SiH.sub.4 gas and GeH.sub.4 gas or a sputtering method has been used in a carrier-producing layer of a solar cell.
However, since these methods use plasma, a-SiGe:H film obtained by these methods has a disadvantage in that the film is injured by charged particles. In addition, a-SiGe:H film obtained by a GD method and a sputtering method has a disadvantage in that the photoelectric characteristics are suddenly reduced with an addition of Ge.
Furthermore, since the GD method requires GeH.sub.4 gas at a ratio of GeH.sub.4 to SiH.sub.4 of 1 or less for the production of a-SiGe:H of 1.6 eV, it has been necessary to use a large amount of expensive GeH.sub.4.
On the other hand, a photo CVD method, in which the injury of a film due to charged particles is reduced in comparison with that in the GD method, has been also used for the production of a-Si:H film, but the production of a-SiGe:H film by said method and the use of that film in a carrier-producing layer of a solar cell have not been known much.
Although, as above described in detail, a-Si has been watched with interest for a material of a thin film solar cell and extensively investigated in view of various kinds of interesting characteristics thereof, a-Si having a sufficient conversion efficiency has not been achieved yet and thus, the development of a practically suitable art has been eagerly desired.
For example, in the above described a-Si:H/a-SiGe:H tandem solar cell it seems to be necessary to improve the characteristics of an a-SiGe:H single-layer solar cell in particular. The reason why the conventional article of such kind is inferior in short-circuit current J.sub.sc and long wave length-sensitivity is that if a GeH.sub.4 raw material gas volume rate ratio GeH.sub.4 /(GeH.sub.4 +SiH.sub.4) is increased, an a-SiGe:H film having a reduced inhibit band width E.sub.g is produced. The reason for this seems to be the deterioration of the photoconductivity (at AM 1.0, 100 mW/cm.sup.2) .DELTA..delta..sub.ph and the dark conductivity .delta..sub.d due to the formation of electric defects in Ge. That is to say, it was necessary to give an a-SiGe:H film the reduced E.sub.g and the high long wave length-sensitivity and thus it was necessary that the above described .DELTA..delta..sub.ph was about 10.sup.-4 (.OMEGA..cm).sup.-1 and .delta..sub.d was reduced to an extent of 10.sup.-8 to 10.sup.-9 (.OMEGA..cm).sup.-1.
In addition, photoconductors of inorganic type such as Se, CdS and ZnO or those of organic type such as polyvinylcarbazol have been used as electrophotographic sensitive members.
In view of the fact that a-Si has advantages in that (1) it is less poisonous than the conventional materials; (2) it is more heat resistant than the conventional materials: (3) it has surface hardness higher than that of the conventional materials; (4) it has sufficient sensitivity up to a long wave length range; and the like, the active researches and development of a-Si have been carried out in recent years.
In general, a glow discharge decomposition method has been used in the production of a-Si sensitive member. In this method SiH.sub.4 or Si.sub.2 H.sub.6 gas is subjected to a plasma decomposition in a high-frequency electric field to form an a-Si thin film on a substrate.
In order to give sensitivity up to a longer wave length range for use in a laser printer, attempts have been made to add Ge to a part or the whole of a carrier-producing layer.
This object can be achieved by using GeH.sub.4 as a raw material gas in the above described GD method.
However, a Ge-contained layer obtained by the GD method has the following disadvantages:
(1) The sensitivity in a long wave length range, which is required for a sensitive member for use in a laser printer, can not be stably obtained.
(2) Even though long wave length sensitivity is possessed immediately after production, the sensitivity is reduced with an increase of the number for times of using the sensitive member.
The reason of such a reduction of the sensitivity of a sensitive member seems to be an increase of the density of the defect-level in a photoconductive layer due to the corona-charge, the exposure and the like.
Next, amorphous films formed of materials such as CdS.CdSe, Se.As.Te and Si have been used as carrier-producing layers of an image sensor.
However, with an image sensor using these amorphous films it is difficult to increase the reading speed to 10 ms/line or more.
Accordingly, recently an increase of the reading speed by using an a-Si:H film and an a-SiGe:H film as a carrier-producing layer has been investigated.
However, it is the production of such an a-Si:H film and an a-SiGe:H film by the GD method and the sputtering method using SiH.sub.4 or Si.sub.2 H.sub.6 or a mixture of GeH.sub.4 gas and SiH.sub.4 or Si.sub.2 H.sub.6 that has been investigated.
However, since said methods all use plasma, an a-SiGe:H film obtained by these methods has a disadvantage in that the film is injured by charged particles.